After bending and installing the rebar, reinforcing grid and vapor barrier, it was now time to pour the foundation slab for our diy backyard wood-fired pizza oven. As I posted about last week, I shrunk the foundation slab down to be 72" wide and 80" front-to-back. It is 5.5" thick (2x6's for concrete forms). This is sitting on top of six inches of 1/2" compacted gravel and a 48" deep of large rock drywell.
I used an online concrete estimating calculator tool and when you put those dimensions in, it says that I needed 42 60# bags. I have to pour two slabs - this foundation slab and then, once I build the stand, an insulated hearth. Each approximately the same size. When adding those two up, I ordered 85 60# bags and had them delivered. I opted for 4000 PSI concrete - as a little bit of an upgrade over the baseline concrete.
Knowing that the labor involved in this pour is in two parts, I opted to split them up: first...hauling the concrete back to the project (from the garage). And second...doing the actual pour.
On day one, I set up a pallet near the project and began to stack the 42 bags. Once that was done, I began to try to figure out how to best set up the mixer so it would 'pour' right into the frame and I wouldn't have to do any extra labor. I decided on a setup where I would create a platform that the MudMixer could sit on about half-way back. See below for both the pallet of concrete and the frame to support the mixer.
On top of the frame, I laid down some 3/4" plywood as the platform - see below. I also added a couple of scrap pieces to serve as a ramp to get the mixer up on top of the project.
I rented the MudMixer for the one-day job. It fit into our minivan with the back seats all the way down. I wheeled it down to the project then pulled over the garden hose. I had a 10 gauge extension cord (100 feet) that I bought when we had our backyard COVID-era Joe Dirt above-ground pool back in 2020. This MudMixer requires a heavy-duty extension cord.
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| The MudMixer allowed me to pour a 72" x 80" 5.5" thick slab with 46 bags in 90 minutes. |
This was my first time pouring a concrete slab and my first time using the MudMixer. I can't recommend it enough. For me, a DIY'er, who was doing this job by myself, it was the perfect tool for the job. The MudMixer has an integrated 'bag breaker' and a hopper that holds more than two 60# bags of concrete mix at once. It also has a dial that allows you to increase/decrease the water that is being provided as part of the mix. The MudMixer has a pivot point, so you can twist/turn the output along the front of the machine, allowing for a pin-point drop of the mixed concrete. This made pouring 46 bags of concrete a truly one-man job and I was able to get through the 46 bags in about 90 minutes. 90 minutes!
It wasn't long before the slab was poured and I was able to move-on to finishing. This slab will be barely viewed, so I didn't kill myself with a perfect finish.
The official plans from Forno Bravo include this note about pouring this reinforced foundation slab:
Remember that while the slab has to be square, level, plumb, and structurally sound to get your oven off on the right foot, it will never be seen. It is worth saving your best finish work for later in the project.
As I said...this is my first time finishing concrete, so that meant there's A LOT of learning when it comes to the tools. I started with a Magnesium float - which opens up the pours, smooths out the aggregate and gives a smooth finish. Once it set-up a little bit more, I went over it with a stainless finishing trowel. The hard part (for me) was knowing WHEN the right time is to work the concrete. It felt TOO wet for a while, then TOO hard to change. I tried using an edger, but it was sort-of 'tearing' the concrete corner and not providing a smooth corner, so I just left it be and didn't put on a curved/shoulder corner. Here, below, is what the slab looks like after I finished:
The night of the pour, it seemed that the concrete was set-up enough to take on some water. So, I sprayed it down with the hose to keep the surface wet. Everything I've read online says to keep the concrete surface wet a few times per day over the first week to help ensure a slow cure and keep the concrete from being brittle. That night, before I went to bed, I laid down a 3 mil plastic sheet on top of the recently-sprayed-down concrete slab to try to help retain some moisture. See below for the slab - covered in a plastic sheet - on day two:
What happens next? I'm going to let the slab cure for a week-or-so, then begin the stand build. I built a LEGO model that I'm using to guide my build.
My plan calls for the stand to be 4 blocks wide x 4.5 blocks deep - 62.5" wide x 70.312" deep. And, I'll be stacking up the cinder blocks to be four full blocks and one half-block tall. The top course will be bond-beam blocks that allow for rebar to be run *through them*. I bought three bags of Type S mortar to level the first course of the block and then I'm planning on dry-stacking the rest of the courses. From there, I'll be filling every-other-core and trying to tie the stand and slab and hearth together with rebar.
I'm thinking that I'll wait seven-or-so days before I start to put the blocks on top of the slab.
I have the block, mortar, rebar and post-setting concrete to fill the cores. Just need a torpedo level, a concrete-cutting disc for the angle iron and a chalkline to set the square.
